I'm typing off the cuff here, so I don't know how well this will stand up to scrutiny--and I certainly haven't been precise in my use of these terms in the past--but I think engagement is about action and detachment is about emotion.
Chatting with my husband about our day is an act of engagement. Not getting sad or stressed about not knowing if he is feeling emotionally connected to me is detachment.
Of course there are actions we take to help us detach--we meditate, pray, distract ourselves by GAL, etc.
It is tricky, absolutely. And different situations have different optimal levels of engagement. A spouse who has asked for space is different from a spouse who feels disconnected due to years of neglect.
I'd love to hear other people's thoughts.
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16